1. Field of the Invention
The present invention relates to transceivers, and more particularly to an infrared digital transceiver having an isolated analog output.
2. Description of the Related Art
Conventional digital infrared (IR) transceivers provide only a digital output. The receiver component of the transceiver converts infrared optical pulses into discrete electrical pulses for downstream processing.
FIG. 1 illustrates a conventional IR transceiver receiving chain. A photodiode (unreferenced) such as a PIN diode converts the optical energy from an infrared optical signal to an electric current. An amplifier 14 converts the electric current to voltage and magnifies the voltage. The output (analog signal) of the amplifier 14 is fed to a comparator input and a threshold voltage is applied to the other input of the comparator 16.
When the analog output signal of the amplifier 14 is greater than the threshold voltage, the comparator 16 emits a digital pulse. If the analog signal amplitude is smaller than or equal to the threshold voltage, the comparator output remains low. Thus, the comparator 16 hides small signal information within the analog profile of the optical signal output from the amplifier, including base line voltage, noise variance, system gain, etc. Thus, these parameters are inaccessible and untestable.
Hence, it is very difficult to apply digital signal processing (DSP) technology to IR communication in this conventional IR transceiver receiving chain. DSP is a very powerful method of improving communication quality, sensitivity and immunity, and the advantages of DSP are well established in other types of communication systems. However, DSP usually requires an analog signal in order to effect the analog-to-digital conversion, and a conventional IR transceiver is incapable of providing an analog signal without risking disruption of the operation of the receiver chain.
Furthermore, the conventional IR transceiver is difficult to test and debug. The analog component of the receiver chain is the core of an IR transceiver. However, because the analog signal is not accessible, when the device malfunctions or experiences product quality problems common diagnostic procedures are largely unavailable because the analog signal output of the amplifier is untestable.
Therefore, it would be advantageous to add an analog output to the receiver chain in an IR transceiver. However, since in an integrated circuit design the load capability of an amplifier is limited due to restrictions in the chip die size, this is significantly more complex than merely adding a connection from an analog output pad to the amplifier output. A direct connection from the amplifier output to an analog output pad would add an additional load on the amplifier output which would affect the receiver chain performance. The extra load results not only from the parasitic capacitance of the connection trace and the pad, but also from the input impedance of external components such as a DSP device or measurement instrument. If the additional load is too heavy, the IR transceiver performance will be degraded. In extreme cases (e.g., where a large capacitance is added to the amplifier output), a conventional IR transceiver will oscillate and system failure will result.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide an IR transceiver having a receiver chain with an isolated analog output, and a method of implementing an analog output in an IR receiver chain.
According to the invention, a transmission gate and unity gain buffer are interposed between the amplifier output and an analog output pad. The transmission gate behaves as a switch, passing the analog output signal to the analog output pad only when an analog output is required. For example, to use the analog output signal for digital signal processing, for a relatively strong signal, it is unnecessary to use digital signal processing and the transmission gate remains off. For a relatively weak signal, the transmission gate is turned on, thereby passing the analog signal to the unity gain buffer which provides an analog signal output to the DSP device. Thus, the transmission gate minimizes the effect of the isolated analog output, and further serves as a switch and multiplexer which allows a single isolated analog output to be connected to different test points through the use of multiple transmission gates.
The unity gain buffer preferably has a high input resistance, a low input capacitance, a unity gain and a low output impedance. Thus, the load of the DSP device or measuring instrument on the analog output pad is isolated by the unity gain buffer and the transmission gate, and does not affect the performance of the receiver chain.
In the preferred embodiment, the bandwidth of the transmission gate and the buffer is wider than that of the receiver chain, and since the transfer rate of the transmission gate (when active) is unity and the transmission rate of the buffer is unity, the analog output pad provides an analog signal substantially identical to the analog output of the amplifier.
In a first aspect, the present invention provides an isolated analog output circuit for a photoelectric transceiver having a photodiode emitting a current proportional to an intensity of an optical signal to produce an analog electrical signal, with the analog electrical signal being amplified by a photodiode signal amplifier to generate an analog input signal to the circuit. The circuit includes a buffer coupled to an analog output terminal, the buffer including a differential amplifier having a very high input impedance and substantially a unity gain. When the analog input signal is coupled to an input of the differential amplifier, the buffer outputs an analog output signal substantially identical to the analog input signal while isolating the photodiode signal amplifier from a load on the analog output terminal.
In a further aspect of the isolated analog output circuit, the differential amplifier includes a first symmetrical transistor pair including an input transistor and an output transistor having a common current source or impedance for signal coupling and operating point setting, the input and output transistors passing a current in direct proportion to an amplitude of an input voltage of the input transistor, a second symmetrical transistor pair including a pair of transistors, forming a current mirror and an active load of the first symmetrical transistor pair, connected between a voltage supply and the first symmetrical transistor pair, and an output stage including an output stage transistor and a resistor forming an impedance transfer and feedback network to the output transistor, an input terminal of the output stage transistor being connected to an output terminal of the first symmetrical transistor pair and an output terminal of the output stage transistor being connected to ground through the output stage resistor and to an input terminal of the output transistor of the first symmetrical transistor pair and to the analog output terminal. When the analog input signal is applied to the input terminal of the input transistor, a current flows through the input transistor and the second symmetrical transistor pair raises the output terminal voltage of the output transistor to maintain substantially identical currents through the input transistor and the output transistor, thereby forcing a current through the output transistor to match the current through the input transistor and providing an output analog signal substantially identical to the input analog signal.
In another aspect of the present invention, a photoelectric transceiver has a photodiode emitting a current proportional to an intensity of an optical signal to produce an analog electrical input signal, having an isolated analog output circuit. The output circuit includes a buffer coupled to an analog output terminal, the buffer including a differential amplifier having a very high input impedance and substantially a unity gain. When the analog input signal is coupled to an input of the differential amplifier, the buffer outputs an analog output signal substantially identical to the analog input signal while isolating the photodiode signal amplifier from a load on the analog output terminal.
In a further aspect of the photoelectric transceiver, the differential amplifier includes a first symmetrical transistor pair including an input transistor and an output transistor having a common current source or impedance for signal coupling and operating point setting, the input and output transistors passing a current in direct proportion to an amplitude of an input voltage of the input transistor, a second symmetrical transistor pair including a pair of transistors, forming a current mirror and an active load of the first symmetrical transistor pair, connected between a voltage supply and the first symmetrical transistor pair, and an output stage including an output stage transistor and a resistor forming an impedance transfer and feedback network to the output transistor, an input terminal of the output stage transistor being connected to an output terminal of the first symmetrical transistor pair and an output terminal of the output stage transistor being connected to ground through the output stage resistor and to an input terminal of the output transistor of the first symmetrical transistor pair and to the analog output terminal. When the analog input signal is applied to an input terminal of the first symmetrical transistor pair, a current flows through the input transistor and the second symmetrical transistor pair raises the output terminal voltage of the output transistor to maintain substantially identical currents through the input transistor and the output transistor, thereby forcing a current through the output transistor to match the current through the input transistor and providing an output analog signal substantially identical to the input analog signal.
In a further aspect of the invention, a transmission gate for selectively blocking or passing the analog input signal to the buffer is provided. The transmission gate includes at least one transistor which selectively blocks or passes the analog input signal responsive to a level of a gate voltage. In a preferred embodiment, the transmission gate includes a P-type FET and an N-type FET with sources (or drains) of the P-type FET and the N-type FET connected to the circuit input and drains (or sources) of the P-type FET and the N-type FET connected to an input of the buffer, a gate of one of the P-type FET and N-type FET being connected to a switching signal through an inverter. Since the source and drain of a P-type FET and an N-type FET are symmetrical and exchangeable, the connections of the sources and drains can be reversed without affecting the operation of the transmission gate.
In further aspects of the invention, the input and output transistors of the first symmetrical transistor pair respectively include a pair of N-type FETs, a gate of the input N-type FET being connected to the analog input signal, the second symmetrical transistor pair includes a pair of P-type FETs, the gates of the P-type FETs being connected to a drain of the input NFET, the input and output N-type FETs are grounded through a DC current source providing a DC operating current to the source coupled differential amplifier, and/or the output stage includes an N-type FET having a source providing the analog output signal and a feedback voltage to a gate of the output N-type FET.
The present disclosure relates to subject matter contained in Canadian Patent Application No. 2,312,516 filed on Jun. 27, 2000, which is expressly incorporated herein by reference in its entirety.